The present invention relates to a tuning control system of a radio receiver. More particularly, the invention relates to a radio receiver tuning control system wherein a desired station is automatically selected by digital operation.
A conventional radio receiver utilizes the following tuning systems. The radio receiver is tuned to the frequency of a desired station by varying the inductance or capacitance of the tuning circuit by appropriate turns of a tuning knob, or to a frequency selected mechanically from stored or recorded frequency signals. The latter is accomplished by previously storing or recording a plurality of desired station frequency signals in a plurality of push-button mechanisms as the mechanical positions of the inductive element or the capacitive element of the tuning circuit. The latter arrangement is the well known push-button type tuning system.
The push-button tuning system is widely used for a mobile radio receiver, because it is capable of preset tuning. The mobile radio receiver, however, must be compact. For this reason, the mobile radio receiver of the latter type in which the preset operation is carried out mechanically is usually capable of selecting about five stations, or frequencies, since the number of presetting times is restricted. Thus, when the receiving area is widened or changed, due to the movement of the vehicle in which the radio receiver is installed, the preset type push-button mechanism cannot cover the receiving frequencies. Therefore, in such a case, the aforedescribed tuning system is very disadvantageous, since a resetting operation is required by releasing the preset condition.
Furthermore, the automatic tuning receiver is in practical use as a mobile radio receiver. The receiver is sequentially tuned to the frequency signals of stations after an automatic sweep for the receiving frequencies. As a result of the use of a semiconductor variable capacitance element in the tuning circuit, the variable control voltage of the variable capacitance element is changed continuously with a potentiometer.
This type of receiver is very advantageous, since all the broadcasting stations in the receiving area can be selected automatically. However, when the broadcasting stations are located next adjacent each other, a longer time is required in order to receive the desired station frequency, since each station must be received sequentially. Furthermore, the selection switch has to be operated each time the station frequency is received. This is a serious disadvantage of this type of tuning system.
As hereinbefore described, the push-button tuning receiver can immediately select the desired station, but cannot cover the overall receiving area. The automatic tuning receiver can receive the signal over the entire receiving area, but cannot select the station immediately. Thus, these systems have an advantage, on one hand, but have a disadvantage, on the other hand, in mobile radio receivers. This raises problems requiring solution.
The principal object of the invention is to provide a radio receiver tuning control system which functions to immediately select a desired station anywhere in the entire receiving range.
An object of the invention is to provide a radio receiver tuning control system which overcomes the disadvantages of known types of push-button and automatic tuning receivers, but provides the advantages thereof.
Another object of the invention is to provide a radio receiver tuning control system which functions efficiently, effectively and reliably to tune a receiver as desired.
Still another object of the invention is to provide a radio receiver tuning control system which performs, in a frequency sweep operation for tuning, a frequency sweep at a comparatively high speed for frequencies other than the specified frequency and at a lower sweep speed for frequencies near the specified frequency, thereby reducing the frequency sweep time without generating "hunting phenomenon" during the tuning operation.
Yet another object of the invention is to provide a radio receiver tuning control system having digital frequency setting means in which the selecting operation of the desired broadcasting station signal is facilitated by converting the value corresponding to each specified frequency signal into a digital signal at a diode matrix circuit and storing the digital signal.